The present invention relates to a reception quality calculation method, a reception quality calculation apparatus, and to a communication apparatus, and in particular relates to a reception quality calculation method, a reception quality calculation apparatus, and to a communication apparatus in which reception quality is measured as a reception environment, and parameters (for example, CQI) corresponding to the reception environment are transmitted to the transmitting apparatus.
A W-CDMA (UMTS) mobile communication system is a wireless communication system in which circuits are shared among a plurality of users, and as shown in FIG. 6, comprises a core network 1, wireless base station control apparatuses (RNCs: Radio Network Controllers) 2 and 3, multiplex separation apparatuses 4 and 5, wireless base stations (Node B) 61 to 65, and a mobile station (UE: User Equipment) 7.
The core network 1 is a network used for routing within the mobile communication system; the core network can for example be configured using an ATM switched network, packet-switched network, router network, or similar. The core network 1 can also be connected with another public network (PSTN), so that a mobile station 7 can communicate with fixed-line telephone sets and similar as well.
The wireless base station control apparatuses (RNCs) 2 and 3 are positioned as higher-level apparatuses of the wireless base stations 61 to 65, and comprise functions for controlling these wireless base stations 61 to 65 and managing their wireless resources to be used and similar. Functions are also comprised by means of which, during handovers, signals from one mobile station 7 are received from a plurality of subordinate wireless base stations, and the data for which quality is better is selected and transmitted to the core network 1.
The multiplex separation apparatuses 4 and 5 are provided between RNCs and wireless base stations, and separate signals addressed to each of the wireless base stations received from the RNCs 2 and 3, and output the signals addressed to each of the wireless base stations; in addition, control is performed to multiplex signals from the wireless base stations and pass the signals to the RNCs.
The wireless base stations 61 to 63 are controlled by RNC 2, and the wireless base stations 64 and 65 are controlled by RNC 3, while performing wireless communication with the mobile station 7. By existing within the wireless area of the wireless base station 6, the mobile station 7 establishes a wireless circuit with the wireless base station 6, and communicates with other communication apparatuses via the core network 1.
The interface between the core network 1 and the RNCs 2 and 3 is called the Iu interface, the interface between the RNCs 2 and 3 is called the Iur interface, the interface between the RNCs 2, 3 and each of the wireless base stations 6 is called the Iub interface, and the interface between the wireless base stations 6 and mobile stations 7 is called the Uu interface; and a network formed by 2 to 6 apparatuses in particular is called a radio access network (RAN). Circuits between the core network 1 and the RNCs 2 and 3 are used in common by the Iu and Iur interfaces, and circuits between the RNCs 2, 3 and the multiplex separation apparatuses 4, 5 are used in common by Iub interfaces for a plurality of wireless base stations.
The above is an explanation of an ordinary mobile communication system; in addition, HSDPA (High Speed Downlink Packet Access) may be adopted as a technology enabling high-speed downlink data transmission (see Non Patent Documents 1 and 2). Here, a simple explanation of HSDPA is given.
Non Patent Document 1: 3G TS 25.212 (3rd Generation Partnership Project Technical Specification Group Radio Access Network; Multiplexing and channel coding (FDD))
Non Patent Document 2: 3G TS 25.214 (3rd Generation Partnership Project Technical Specification Group Radio Access Network; Physical layer procedures (FDD))
HSDPA
HSDPA employs an adaptive coding and modulation method (AMC: Adaptive Modulation and Coding), and has the feature of enabling adaptive switching between, for example, the QPSK modulation scheme and the 16QAM scheme, according to the wireless environment between the wireless base station and mobile station.
In addition, HSDPA employs the H-ARQ (Hybrid Automatic Repeat reQuest) method. In H-ARQ, when a mobile station detects an error in data received from a wireless base station, the mobile station transmits a resend request (NACK signal) to the wireless base station. Upon receiving this resend request, the wireless base station resends the data, and the mobile station uses both the data already received and the resent reception data to perform error correction decoding. Thus in H-ARQ, data already received can be used effectively even when errors occur, so that the gain of error correction decoding is increased, and consequently the number of resends can be reduced. When an ACK signal is received from the mobile station, data transmission has been successful and resending is unnecessary, and so the next data is transmitted.
The main wireless channels used in HSDPA are, as shown in FIG. 7, (1) HS-SCCH (High Speed-Shared Control Channel), (2) HS-PDSCH (High Speed-Physical Downlink Shared Channel), and (3) HS-DPCCH (High Speed-Dedicated Physical Control Channel).
HS-SCCH and HS-PDSCH are both shared channels in the downlink direction (that is, from the wireless base station to the mobile station); HS-SCCH is a control channel used to transmit various parameters relating to the data transmitted in the HS-PDSCH channel. In other words, HS-SCCH is a channel used for notification of data transmission in the HS-PDSCH. The various parameters may include, for example, address information for the mobile station to which data is transmitted from the base station, transmission bitrate information, modulation method information indicating the modulation method used to transmit data using HS-PDSCH, the number of spreading codes allocated (number of codes), rate-matching patterns for transmitted data, and similar.
On the other hand, HS-DPCCH is a dedicated control channel in the uplink direction (that is, from the mobile station to the wireless base station), and is used when the mobile station transmits reception results (ACK signals, NACK signals) to the wireless base station according to the presence or absence of errors in data received via HS-PDSCH. That is, this channel is used when transmitting reception results for data received over HS-PDSCH. When the mobile station fails in data reception (when the reception data results in a CRC error, or similar), a NACK signal is transmitted from the mobile station, and so the wireless base station executes resend control.
In addition, HS-DPCCH is used by the mobile station, which has measured the reception quality (for example, SIR value) of signals received from the wireless base station, to transmit the reception quality, as a CQI (Channel Quality Indicator), to the wireless base station. In other words, the CQI is information used by the mobile station to report the reception environment to the base station, and takes the values CQI=1 to 30; the CQI for which the block error rate BLER in the reception environment does not exceed 0.1 is reported to the base station.
The wireless base station uses the received CQI to judge the acceptability of the downlink-direction wireless environment, and if satisfactory, switches to a modulation method enabling transmission at faster data rates; that is, if the environment is not acceptable, the modulation method is switched to a slower data transmission method (that is, adaptive modulation is performed). In actuality, the base station has a CQI table which defines formats with different transmission rates according to a CQI value of 1 to 30, uses the CQI table to determine the parameters (transmission rate, modulation method, number of multiplex codes, and similar) according to the CQI value, and uses HS-SCCH to notify the mobile station, while transmitting data over HS-PDSCH based on these parameters.
Channel Structure
FIG. 8 explains timing in the HSDPA system. In W-CDMA, code-division multiplexing is used, and so channels are separated by codes. The CPICH (Common Pilot Channel) and SCH (Synchronization Channel) are both shared channels in the downlink direction. CPICH is used by mobile stations for channel estimation, cell searching and similar, and is a channel used for transmission of so-called pilot signals. SCH comprises, more precisely, P-SCH (Primary SCH) and S-SCH (Secondary SCH), and are channels for transmission in burst mode with 256 chips at the beginning of each slot. SCH data is received by a mobile station performing three-stage cell searching, and is used to establish slot synchronization and frame synchronization, and to identify base station codes (scramble codes). SCH is 1/10 the length of one slot, but is shown as more broad in the figure. The remaining 9/10 is the P-CCPCH (Primary-common control physical channel).
Next, channel timing relations are explained. In each channel, one frame (10 ms) comprises 15 slots, and one frame is of length equivalent to 2560 chips. As explained above, CPICH is used as reference for other channels, and so the frame beginning in SCH and HS-SCCH coincides with the frame beginning in CPICH. On the other hand, the frame beginning in HS-PDSCH is delayed by two slots relative to HS-SCCH and similar; this is in order to enable the mobile station to perform demodulation of HS-PDSCH using the demodulation method corresponding to the modulation method after receiving modulation method information via HS-SCCH. In HS-SCCH and HS-PDSCH, one subframe comprises three slots.
HS-DPCCH is an uplink channel, the first slot in a subframe of this channel is used to transmit ACK/NACK signals, indicating the HS-PDSCH reception result, from the mobile station to the wireless base station after approximately 7.5 slots have elapsed from reception of HS-PDSCH. The second and third slots are used to periodically transmit feedback CQI information to the base station for use in adaptive modulation control. Here, the transmitted CQI information is computed based on the reception environment (for example, the CPICH SIR measurement results) measured over the interval from four slots before CQI transmission to one slot before CQI transmission.
Mobile Station Configuration
FIG. 9 shows the configuration of principal portions of a mobile station of the prior art. Wireless signals transmitted from a base station are received by the antenna and input to the receiver 1. The receiver 1 down-converts the wireless signals to baseband signals, and then performs quadrature demodulation, AD conversion, despreading, and other processing of the baseband signals, and outputs HS-PDSCH symbol signals, CPICH symbol signals, reception timing signals (frame sync, slot sync signals), and similar. The HS-PDSCH channel estimation filter 2 calculates the average value of CPICH symbol signals for the n symbols preceding the current symbol, for example 10 symbols, and for the next 10 symbols including the current symbol, for a total of 20 symbols, and outputs the average value as a channel estimation value in sequence at symbol periods. One CPICH slot comprises 10 symbols, and so the above 10 symbols are equivalent to one slot.
FIG. 10 explains operation of the HS-PDSCH channel estimation filter 2; the first symbol channel estimation value for the current slot# n is the average value of CPICH symbol signals for the 20 symbols which are the first through tenth symbols of the previous slot# n−1 and the first through tenth symbols of the current slot# n. The second symbol channel estimation value for the current slot# n is the average value of the CPICH symbol signals for the 20 symbols which are the second through tenth symbols of the previous slot# n−1, the first through tenth symbols of the current slot# n, and the first symbol of the next slot# n+1. Similarly, the channel estimation value at the tenth symbol of the current slot# n is the average value of the CPICH symbol signals of the 20 symbols which are the tenth symbol of the previous slot# n−1, the first through tenth symbols of the current slot# n, and the first through ninth symbols of the next slot# n+1. In this way, by calculating channel estimation average values for a plurality of symbols on both sides and using the result as the channel estimation value for the center symbol, high-precision channel estimation is possible.
In order to clarify the fact that the channel estimation value for the center symbol is obtained by calculating the average of a plurality of channel estimation values on both sides, FIG. 10 shows channel estimation and channel compensation processing being performed in slot# n. However, in actuality, channel estimation and channel compensation processing are performed in slot# n+1, as shown in FIG. 11.
Returning to FIG. 9, the HS-PDSCH symbol buffer 3 holds HS-PDSCH symbols for one slot interval, and inputs the symbols to the HS-PDSCH channel compensation processing portion 4. That is, HS-PDSCH symbols are input to the HS-PDSCH channel compensation processing portion 4 with a delay of one slot interval, until the channel estimation value is determined. The HS-PDSCH channel compensation processing portion 4 uses channel estimation values to perform channel compensation processing of HS-PDSCH symbol signals with a one-slot delay, as shown in the bottom of FIG. 11. The demodulation processing portion 5 uses channel-compensated symbol signals to demodulate HS-PDSCH symbols, and the decoding processing portion 6 performs error-correction decoding of the demodulated signals; the CRC computation portion 7 performs CRC computation to determine whether errors exist in the decoded results for each block, and if no errors are detected outputs the decoded data and generates an ACK, but if errors are detected generates a NACK, which is input to the HS-DPCCH generation portion 13.
The CPICH channel estimation filter for SIR calculation 8 calculates the average value of CPICH symbol signals for the immediately preceding 20 symbols including the current symbol, and outputs the averages as channel estimation values in sequence at symbol periods. FIG. 12 explains operation of the CPICH channel estimation filter 8; the channel estimation value for the first symbol of the current slot# n is the average value of CPICH symbol signals for the 20 symbols which are the second through tenth symbols of two slots previous, slot# n−2, the first through tenth slots of the previous slot# n−1, and the first symbol of the current slot# n. The channel estimation value of the second symbol of the current slot# n is the average value of CPICH symbol signals for the 20 symbols which are the third through tenth symbols of two slots previous, slot# n−2, the first through tenth slots of the previous slot# n−1, and the first and second symbols of the current slot# n. Similarly, the channel estimation value of the tenth symbol of the current slot# n is the average value of CPICH symbol signals for the 20 symbols which are the first through tenth slots of the previous slot# n−1, and the first through tenth symbols of the current slot# n. The reason why the CPICH channel estimation filter for SIR calculation 8 cannot use CPICH symbol signals for the 20 symbols which are the immediately preceding 10 symbols and the next 10 symbols including the current symbol, as in the case of the HS-PDSCH channel estimation filter 2, is explained later.
Returning to FIG. 9, the CPICH channel compensation processing portion 9 for SIR calculation uses the CPICH channel estimation values for SIR calculation to perform channel compensation processing of CPICH symbol signals, as shown at the bottom of FIG. 12, the demodulation processing portion 10 uses the channel-compensated symbol values to demodulate the CPICH symbols, and the CPICH-SIR calculation processing portion 11 uses the demodulated CPICH symbols to perform well-known SIR calculation processing, and outputs CPICH-SIR values which indicate the reception environment for the mobile station.
The CPICH-SIR-CQI report value conversion portion 12 comprises a CPICH-SIR and CQI correspondence table, as shown in FIG. 13, and determines CQI report values corresponding to input CPICH-SIR values from this table, for input to the HS-DPCCH generation portion 13.
In parallel with the above, the downlink reception timing monitoring portion 14 monitors the downlink timing based on reception timing signals (frame sync, slot sync signals), and the uplink transmission timing management portion 15 inputs transmission timing signals to the HS-DPCCH generation portion 13. The HS-DPCCH generation portion 13 generates HS-DPCCH signals, comprising CQI report values corresponding to the CPICH-SIR values for the previous fourth through first slots (in the example of FIG. 9, the CPICH-SIR for the second and first previous slots) for each subframe, as explained in FIG. 8, and comprising ACK/NACK signals as appropriate; the coding processing portion 16 performs encoding and inputs the result to the modulation processing portion 17. The modulation processing portion 17 performs spreading processing, DA conversion, and quadrature modulation, and the transmitter 18 frequency-converts the baseband signals to RF signals and transmits the signals via the antenna toward the base station. Although not shown, the base station demodulates the HS-DPCCH signals, and based on the CQI report value determines the transport block size, number of multiplex codes, modulation method and similar from the CQI table, and based on these values HS-PDSCH data is transmitted, and resend control based on ACK/NACK signals is performed.
As explained above, by delaying the HS-PDSCH symbols by one slot, the HS-PDSCH channel estimation filter 2 calculates the average value of CPICH symbol signals for a total of 20 symbols, including the 10 symbols immediately preceding the current symbol and the next 10 symbols including the current symbol, and can use this average value as the channel estimation value for the current symbol, so that high-precision channel estimation is possible. On the other hand, the CPICH channel estimation filter 8 for SIR calculation cannot calculate the channel estimation value using the next 10 symbols including the current symbol, as in the case of the HS-PDSCH channel estimation filter 2. This is because the CQI report value must be determined and transmitted for the current slot using the SIR measured based upon three slots' worth of CPICH symbols, which are previous fourth through first slots from the current slot, thereby the CPICH symbols for SIR calculation cannot be delayed for use in channel estimation.
From the above, the CPICH channel estimation filter 8 for SIR calculation calculates the average values of CPICH symbol signals for the immediately preceding 20 symbols including the current symbol, and outputs the average values in sequence as channel estimation values at symbol periods. This means that, as for example the channel estimation value for the first symbol of the current slot# n, the channel estimation value of the first symbol of the previous slot# n−1 is used. For this reason, a channel estimation value not suited to the first symbol of the current slot# n is calculated, and so the precision of the CPICH channel estimation value for SIR calculation is reduced compared with the HS-PDSCH channel estimation value. This effect is particularly prominent in an environment in which the channel estimation result changes in a short period of time due to rapid fading or for other reasons, so that past channel estimation values and current channel estimation values are different. That is, in an environment with rapid fading, the precision of CPICH channel estimation values for SIR calculation is reduced substantially compared with HS-PDSCH channel estimation values, and so the reception quality of CPICH symbols for SIR calculation is degraded considerably relative to the reception quality of HS-PDSCH symbols.
FIG. 14 is a graph showing quantitatively the HS-PDSCH block error rate (BLER) versus fading rate characteristic during fixed-format reception; FIG. 15 is a graph showing quantitatively the CPICH-SIR versus fading rate characteristic; and FIG. 16 is a graph showing the CQI report value versus fading rate when CPICH-SIR is converted to CQI report values using a technique of the prior art. Here, “fixed-format reception” means reception during transmission with the block size, modulation method, and number of multiplexing code channels fixed.
As is clear from FIG. 14 and FIG. 15, as the fading rate increases the CPICH reception quality for SIR calculation is degraded compared with the HS-PDSCH reception quality. As a result, as shown in FIG. 16, during rapid fading, CQI report values are reported to be low compared with the intrinsic CQI report values. As a result, in an environment in which high-quality HS-PDSCH data transmission/reception is possible even using a format with high transmission rate and low error-correcting capability, the base station transmits data to the mobile station over HS-PDSCH using a format with a low transmission rate and high error-correcting capability. As a result, the HS-PDSCH block error rate BLER is substantially lower than the stipulated value of 0.1; that is, quality is excessively high, and the communication system throughput characteristic is degraded.